Biological marker compounds as indicators of the depositions! history of the Maoming oil shale

Advances in Organic Geochemistry 1985 Org. Geochem. Vol. IO, pp. 927-941. 1986 Printed in Great Britain. All rights reserved

0146-6380/86 $3.00 + 0.00 Copyright 0 1986 Pergamon Journals Ltd

Biological marker compounds as indicators of the depositional history of the Maoming oil shale

SIMON C. BRASSELL’, GEOFFREY EGLINTON’ and Fu JIA Mo2 ‘Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantock’s Close,

Bristol BSS ITS, U.K. *Institute of Geochemistry, Academia Sinica, Guiyang, Guizhou Province, The People’s Republic of China

(Received 21 November 1985; accepted 18 March 1986)

Abstract-The Eocene Maoming oil shale from Guangdong Province occurs as a laterally uniform stratigraphic section, typically 20-25 m thick, from which the aliphatic hydrocarbon constituents of six representative samples were investigated using GC and C-GC-MS. The sediments evaluated included the basal lignite, a vitrinite lens from the overlying claystone, and four intervals from the massive oil shale bed. As expected, the lignite and vitrinite differ markedly from the oil shales. The lignite is dominated by bacterial hopanoids and components of higher plant origin, including C,, steroids and triterpenoids such as oleanenes. Visually, the oil shale samples show corroded and degraded phytoclasts, spores, wispy particles of fluorescent organic material attributable to dinoflagellates and, especially in the uppermost sample, colonial algal bodies. The distributions of biological markers in the oil shales show many features in common, notably a dominance of dinoflagellate-derived Cmethylsteroids, and a significant proportion of higher-plant derived n-alkanes with marked odd-over-even carbon number predominance. Overall, they exhibit several features that resemble characteristics of the Messel shale. The hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales. The aliphatic hydrocarbons of the uppermost shale are dominated by a number of C,, and C,, botryococcane homologues and other unusual branched alkanes possibly derived from green algae. All of the samples are immature. Overall, molecular and microscopic examination of the stratigraphic succession of the Maoming oil shale suggests a shallow, lacustrine environment within which peats were deposited. This lake subsequently deepened to support abundant algal populations, especially dinoflagellates. culminating in a dominance of botryococcoid algae.

Key words: biological markers, Borryococcus, Chinese oil shales, dinoflagellates, hopanoids, lignite, 4-methylsteroids, oil shales, steroids, vitrinite

INTRODUCTION Within the field of molecular organic geochemistry the use of biological marker compounds in the assem- bly of the maturity of sediments and petroleums is now firmly established (e.g. Mackenzie et al., 1980; Mackenzie, 1984; Seifert and Moldowan, 1978). Simi- larly, their value in the identification and evaluation of petroleum biodegradation is clearly recognized (e.g. Seifert and Moldowan, 1979; Connan, 1984). Less well documented, but equally significant, is the appreciation and utilization of the information contained within biological marker distributions for the characterisation, differentiation and assessment of depositional environments. In particular, the general objective of distinguishing ancient marine and non- marine petroleum source rocks (e.g. Moldowan et al.,

1985; Brassell et al., in press) is complemented by the secondary, but important goal of recognizing whether an ancient marine environment was open, deltaic or restricted and, similarly, whether lacustrine systems are freshwater, brackish, saline or evaporitic.

The realization of the potential of biological markers for palaeoenvironmental assessment requires knowledge of (i) compounds that are diagnostic of specific types or families of organisms (e.g. Brassell et al., 1983; Brassell and Eghnton, 1983a and 198%

(ii) the survival and occurrence of such compounds in recent marine and lacustrine sediments (e.g. Simoneit, 1978; Brassell et al., 1980; Brassell and Eglinton, 1983a and 1986; Cranwell, 1982), (iii) the diagenetic fates of diagnostic compounds (e.g. Mackenzie et al., 1982), (iv) the influences of evolutionary changes in the origins of sedimentary organic matter (Brassell and Eglinton, 1983b) and (v) an integration of biological marker data with the complementary evidence provided by visual description of the insoluble organic matter (e.g. Combaz, 1980; Alpem, 1980) and by isotopic studies (e.g. Schoell, 1984). In this paper, the composition of biological markers in a suite of samples from an Eocene lacustrine sediments sequence in southern China is evaluated with a view to the assessment and interpretation of its depositional history.

Two oil shale horizons outcrop in the vicinity of Maoming in Guangdong province, some 400 km to the southwest of Guangzhou (Canton; Fig. la). The older, Eocene, deposit, which outcrops over cu. 50 km, is studied herein. It lies unconformably on Cretaceous red sandstones and is overlain by Neo- gene sandstones, gravels, conglomerates and clays, including the second oil shale horizon (Fig. lb). The Eocene oil shale strata vary between 2-38 m in thick-

927

928 SIMON C. BRASSELL el al.

The Peoples’s Republic of China

(a)

-. / Vitrinite

Lignite

Fig. I. (a) The location of Maoming in Guangdong Province in the People’s Republic of China. (b) Sketch map indicating the outcrop of the Eocene and Neogene Maoming oil shales together with a cross-section

of the outcrop from which the six samples were collected.

Biological markers in Maoming oil shale 929

ness, but are normally 20-25 m. They are actively mined for retorting, yielding up to 11% oil with an average content of c. 7%. The poorer yields tend to be those obtained from the lower 5 m of the oil shale bed. These oil shale beds tend to be uniform laterally, dipping to the northeast at 4-12”. They are bounded to the North, South and northeast by faults.

The visual description of the kerogen and the biological marker distributions of six representative samples from the Eocene oil shale sequence at Mao- ming are considered herein. The data obtained from these two procedures are assessed independently and compared and contrasted in order to describe and characterize the individual samples, leading to a scenario of the depositional history of the Maoming oil shale.

Sampling

EXPERIMENTAL

Six samples were selected from the stratigraphic succession of the Maoming oil shale at an outcrop where it is mined for retorting (Fig. lb). These consisted of: M-4 from the basal lignite of the succession; M-5 from a vitrinite lens within the overlying claystone; M-l, M-2 and M-3 from near the base, middle and upper part, respectively, of the oil shale sequence; M-M was taken from the uppermost lighter coloured oil shale horizon.

Organic carbon, virrinite reflectance and clay content

The organic carbon contents of samples ground in a Tema disc mill were determined using a Perkin Elmer 240 CHN analyser. Vitrinite reflectance measurements for the lignite and the oil shales were made in China and the composition of clays in the M-2 oil shale was determined by X-ray diffraction.

Exlraction, fractionation and analysis of aliphatic hydrocarbons

All samples were extracted by Soxhlet (CH,CI,/MeGH), and their aliphatic hydrocarbons were isolated by standard

M-M

M-3

M-2

M-l

M-5

M-4

Table 1. Organic carbon contents and vitrinite reflectance of Maoming oil shale sequence samples

Lithology Code Org. c (%) Ro (%I

f M-M 11.0

Oil shale i

M-3 14.3 M-2 13.2 ” ld “._ .

1 M-l 7.0

Vitrinite M-5 61.3 Lignite M-4 49.9 0.38

chromatographic procedures and analysed by gas chromatography (GC) and computerised gas chromatography- mass spectrometry (C-GC-MS), according to previously published methods (e.g. Brassell ef al., 1984). Biological marker compounds were identified by reference to standard compounds, literature spectra or by spectral interpretation leading to their qualitative assessment. All of the figures illustrating the distributions of a given class of biological markers in individual mass chromatograms are shaded independently; hence, there is no correlation between similarly designated peaks in different figures.

RESULTS

Organic carbon contents, vitrinite reflectance and clay com- posilion

The organic carbon contents of the samples are all greater than 10% (Table I), except for M-l. M-l, however, is still sufficiently enriched in organic matter to warrant the de- scriptor ‘oil shale’. Its comparative leanness is consistent with the fact that the lowest 5 m of the oil shale provides the poorest oil yield on retorting. The organic carbon contents of the lignite and vitrinite suggest both their impurity and immaturity. The vitrinite reflectance values for both the lignite and the oil shale (Table 1) confirm the immaturity of these strata. The analysis of M-2 using X-ray diffraction revealed a clay composition of 70% kaolinite, 17% mixed layer montmorilIonite-illite, 10% illite and 3% mixed layer montmorillonite-chlorite, also suggestive of general immaturity.

Visual characteristics of samples

All six samples were examined visually first under white light, and subsequently under “blue” (ultraviolet and up to

Oil shale

fitrinite

Lignite

Visual description

Fig. 2. Schematic summary of the relative amounts of visual features of the samples recognised by microscopic examination of their kerogens. This figure is intended to provide a comparative indication of the relative contributions of each specific component to each sample. This diagram does nor represent

an accurate quantified picture.

930 SIMON C. BRA~~ELL er al.

410 run) light to assess their fluorescence properties. The petrographic characteristics revealed in these investigations are summarised in Fig. 2.

Oil shales. The shales are all fine grained, non-pyritic and heavily impregnated with hydrocarbon material which is strongest in M-3, decreasing in intensity to M-2 and M-l. Their contents of vitrinite and inertinite, which are corroded and degraded, are extremely low. Under “blue” light they fluoresce strongly. M-l contains a few wispy bodies too small to resolve and probably attributable to dinoflagellates, and a few spores and colonial algae. M-2 is rich in spores and in colonial algal bodies, with only a trace of fluorescing wisps. M-3 shows a strong background fluorescence and is rich in fluorescing wisps, but contains only trace amounts of spores and no algal colonies. M-M contains few phytoclasts, namely a few spores and a trace of vitrinite and wispy organic material. Its dominant feature is a strong yellow fluorescence arising from its high content of algal colonies. These, like those of M-l and M-3, are of Borryacoccus-type, but lack the fan-like filamentous structure of Borryococcus.

Lignite and uirrinite. The lignite (M-4) contains about 10% spores, 20% semifusinite and 70% vitrinite, with some spheroidal bodies that appear to be polymerised resin. It also has occasional clay inclusions. The vitrinite sample (M-5) is wholly vitrinite with dessication cracks and some resin bodies.

Biological markers

The various samples showed major differences in their distributions of aliphatic hydrocarbons, as seen in the annotated reconstituted ion chromatograms (RIC) of the lignite and two of the oil shales (Fig. 3). The most prominent featpres lie in the polycyclic region of these RIC traces where a dominance of botryococcanes, Cmethylsteranes and hopanoids is observed in the M-M oil shale (Fig. 3a), the M-3 oil shale (Fig. 3b) and the lignite (M-4; Fig. 3c), respectively. The composition of these and other compound classes in the various samples are considered below.

n -Alkanes and branched alkanes. n -Alkanes are prominent components of the aliphatic hydrocarbon distributions in all the samples. They exhibit a marked odd/even predominance in the C,,-C,, region, with a secondary maximum at C,, or C,, for the oil shales (Figs 3a and 3b). By contrast the lignite (M-4, Fig. 3c), and also the vitrinite (M-5), show less predominance of the odd-numbered n-alkanes, with their distributions maximizing at C, and q,, respectively. The relative proportion of lower n-alkane homologues is higher in the oil shales than in the lignite and vitrinite samples. Hence, the n-alkane distributions of the latter are significantly less bimodal. In addition to n-alkanes, mass fragmentography of m/z 85 revealed that the lignite also possessed significant amounts of iso- and anreiso-alkanes in the Cr.&, range with Lro-C,, and iso-C, most prominent members (cf. Fig. 3c) and with iso-alkanes more abundant than their anreiso-counterparts.

Acyclic isoprenoid and related alkanes. C,, and C,s-Cl0 regular acyclic isoprenoids (C,,i, C,,i, Pr and Ph, respectively, in Fig. 3) were recognized as significant components of all the samples. Minor amounts of C,, and C,r acyclic isoprenoid alkanes were also detected. Pristane (Pr; C,,) was the major homologue, dominating phytane (Ph; C,,) in all cases, ‘but especially in the lignite (M-4; Fig. 3~). In the vitrinite, M-S, the C,, component was particularly abundant. Also pristane and phytane greatly dominated the lower n-alkanes (e.g. C,, and C,,) in both the lignite (Fig. 3c) and vitrinite. Higher (i.e. > C,) acyclic isoprenoid alkanes were only detected in the M-3 and M-M samples where three components eluting in the n-C, to n-C,, region were assigned as lycopane (peak c in Fig. 3b) and two other, possibly C,, acyclic isoprenoids of unknown structure (peaks a and b in Fig. 3b). The first of these unknowns (peak a in Figs 3a and 3b) was previously

reported, together with lycopane (Kimble ef al., 1974). as a constituent of the Messel shale (Kimble, 1972). Its mass spectrum shows ion doublets that indicate a regular isoprenoid structure with methyl substitution at C-2, C-6, C-10, C-14, C-18, C-22 and C-26. The absence of a molecular ion precludes confirmation of its carbon number, but a CJ9 (or Ci,,?) compound seems more likely than C, in view of its GC coelution with n-C,,. The second unknown. ehtting between n-C, and n-C,,(peak b in Fig 3b), pas: sesses a mass spectrum with prominent ion doublets at m/z 252/253 and m/z 308/309, which serve to confirm its branched character but provides few clues to its precise structure.

The M-M oil shale differed most markedly from the other samples in respect of the presence of several components structurally related to acyclic isoprenoids which eluted in the n-C,, to n-C,, region. Six distinct components were recognized by C-GC-MS following analysis on both OV-1 and SP-2250 GC phases (Fig. 4). The dominant component is a C,, botryococcane, identified by direct comparison of its mass spectrum (Fig. 5b) with that of botryococcane (Moldowan and Seifert, 1980; Fig. 5c) and other botryococcane and botryococcene homologues (Metzger and Casadevall. 1983: Metzzer ef al.. 1985). It shows (Fie. 5b) ion doublets arising from cleavages an-d rearrangements at the branched positions (i.e. m/z 434/435, 280/281 and 238/239) suggesting an alkylation pattern different from that of the C,, alkene isolated from Borryococcus (Metzger er al., 1985), namely one with methylation at C-3, C-16 and C-20 (inferred from ion doublets at m/z 434/435, 294/295 and 2241225 of the alkane produced by hydrogenation), rather than C-3, C-7 and C-20. [Such methylation is additional to the IO-ethyl-2,6,10,13.17,21-hexamethyldo- cosane skeleton common to all botryococcanes (Metzger and Casadevall, 1983).] Two other, less abundant components with mass spectra identical to that of the major C,, botryococcane were also recognized (Fig. 4). These compounds are perhaps structural or sterical isomers. An earlier ehtting component possesses a mass spectrum (Fig. Sa) consistent with that expected for a C,, botryococcane with methylation at C-3. Again, this position of methylation is different from that seen for a C,, botryococcene in Borryococcus (Metzger et al., 1985).

Apart from the botryococcanes, M-M also contains two other branched alkanes with virtually identical mass spectra (Fig. 6b) which correspond to that expected for a C,, homologue of 2,6,10-trimethyl-7-(3-methylbutyl)dodecane (Fig. 6a; Yon et al., 1982; Rowland er al., 1985), namely 2,6,10,14,18- pentamethyl-7-(3-methylpentyl)nonadecane. The mass spectra and GC retention times of these two isomeric components cannot determine whether they differ in structure (i.e. methylation patterns) or steric configuration. None of these unusual acyclic alkanes was detected in the other samples, nor were their C&, and G, homologues (Yen et al., 1982; Rowland et al., 1985).

Hopanoids. The distributions of hopanes and hopenes, as exemplified in m/z 191 mass chromatograms (Fig. 7), for all the oil shales, especially M-2, M-3 and M-M, are generally similar. They all differ, however, in several respects from those of the lignite and vitrinite. In particular, the lignite contains higher proportions of h’701)-hopenes. Ai3(i*)-neohooenes and 22R 17a(H),21/3(H)-homohopane . (C,,; Fig. 7cj than the upper oil shale horizons. Overall, it is the relative abundance of the individual hopanoids that varies from sample to sample, since essentially they all contain similar series of hopenes and hopanes. For example, the m/z 191 chromatogram of both the M-l oil shale and the vitrinite (M-5) are dominated by 30norneohop- 13(18)-ene (C,). The relative proportion of 17/3(H),21/3(H) isomers of the hopanes is lower in the M-M oil shale than in the other samples. Also, the proportion of C, components is markedly higher in the lignite than in the oil shales.


(a)

C,,

botry

ococ

cane

“-c26

n-

C27

I n-

C25

M-M

oi

l sh

ale

1 C

S3

botry

ococ

cane

s

n -c

28

n-c2

9

(b)

C,,

botry

ococ

cane

s

C3,

bot

ryoc

occa

ne

- -_

+ Ti

me

Fig.

4.

A pa

rtial

R

IC c

over

ing

the

n-C

,, to

n-C

,, re

gion

fro

m

C-G

C-M

S an

alys

es o

f th

e al

ipha

tic

hydr

ocar

bons

of

the

M-M

oi

l sh

ale

usin

g:

(a)

30 m

SP-

2250

co

lum

n w

ith

H,

carri

er

and

(b)

25 m

OV-

I co

lum

n w

ith

He

carri

er.


(a) 57 100.0~ ,

r 25.0X (M-MI

(b)

(cl

71 224 C3, botryococcone

85 50.0-

99 211 266

‘27 239 155 183 28’

1 I I II I, I .I .I I. 1. 1 I, I. 309 I I 337

50 I , _.

I 100 150 200 250 300 350 I I 400 450

7’ r 8.0X m/z

lOO.O- 57 85 238

CJ3 bot ryococcane

99 50.0- I13

I41 21 I 280

169 225

4 .L ,‘I ,I’ ’ 197 I 253 1 .I. 12% I . 327 . . 351 . . 434 I I I

loo.o--M5771

loo 150 I I

200 250 300 350 400 450

r 15.0x m/z

Botryocaccane

85 238

50.0- 211 99 294 II3 225

I41

I I Ir L I I I ‘y 197 253

LI I1 1. 1309 I.. 337 365 448 I I I I I

50 100 rii&Gy 250 300 350 400 450 m/z

Fig. 5. Mass spectra (40 eV, Finnigan 4000) of aliphatic hydrocarbon constituents of the M-M oil shale (cf. Fig. 4): (a) a C,, botryococcane (lo-ethyl-2,3,6,10,13,17,21-heptamethyldocosane) and (b) a C,, botryococcane (10-ethyl-2,3,6,7,10.13,17,20,21-nonamethyldocosane) together with (c) authentic bot-

ryococcane (10-ethyl-2,3,6,7,10,13,16,17,20,21-d~methy~docosane).

I- -. 1 ^.. ^ I (a) 1 note\ ram uli seep 1

loo.O- 57 r 12.0x

168 71

50.0- 85

Cm branched alkane

99 M?

I,. ] , Il. 155 I I loo 127 197 282

if? 23 I I I I

[ 100 150 200 250 300 350 m/z

(bl 4 Inno, 5? ?’ lr

r 40.0x

I ,

i b.

336

7’ 200 2&l 3&l 340 m/z

Fig. 6. Mass spectra (conditions as in Fig. 5) of (a) 2,6,10-trimethyl-7-(3-methylbutyl)dodecane from Rozel Point crude oil and (b) one of the two isomeric components of the M-M oil shale thought to be

2.6,10,14,18-pentamethyl-7-(3-methylpentyl)nonadecane.

934 SIMON C. BRASSELL et al.


C 29 - sterones

C27 - steranes -- j-Jzq

C 28 - sterones pi5iGxq

,.... I..,.: ..,...: ,...a ‘.,....,‘.“,.. c -A,+. +auh.~~.J!J .$ ,JJl&b_

(b) M-3 oil shate

GO - 4 - methyl

steranes

C&9

4-methylsteranes

-k Time

Steranes ,

50 (HI 5p(H)

50(H), 140(H), 171x(H)- ::;

13 ..: . . ..‘. 4a - methyl

B

tzl f&?(H), 14a(H), 17a(H)-

El 4B-methyl

I L

Fig. 8. Annotated partial m/z 217 chromatograms illustrating the distributions of steranes and 4-methylsteranes among the aliphatic hydrocarbons of: (a) M-M oil shale and (b) M-3 oil shale.

Sferanes. The sterane contents of the samples are dominated by 5x(H)- and S/I(H)-steranes (seen in m/z 217 mass chromatograms; Fig. 8). No 20s nor 14/?(H),17j?(H)- steranes were detected in any of the samples. The sterane profiles of the samples are highly complex for the lignite and vitrinite due to minor responses from their markedly more abundant hopanoids. Representative chromatograms for two of the oil shales are illustrated (Fig. 8). In their m/z 217 chromatograms Cmethylsteranes are also evident, most markedly in the M-3 (Fig. 9b) and M-l samples. The carbon number distributions of the steranes in the oil shales are generally similar with C,, and C,, members in roughly equal

abundance (Figs 8 and 9). Higher relative amounts of C,, components, however, were found for the lignite and vitrinite samples.

4Methylsreranes. The Cmethylsteranes observed in the m/z 217 chromatograms can be batter seen in m/z 231 chromatograms, except that coeluting A”“‘)-hopenes can interfere with the peaks for the G, and C, components. This problem is most marked for the lignite. Similar distributions of C,r-Cu, 4-methyl-5a(H)-steranes were recognized in all the samples, present as 4a- and 48-methyl isomers in roughly equal proportion. Smaller amounts of 4a- and 4/?-methyl-Sfi(H)-steranes were present. The

936 SIMON C. BRASSELL er al.

(a)

Carbon number distribution

( b)

Fig. 9. Composition of C,,, C, and (&,: (a) 20R 5a(H),l4a(H),17a(H)-steranes and (b) 20R diacholest- 13(17)-enes in the six samples.

C, components [4a- and 4/J-methyl-5a(H)- and 5b(H)- steranes] greatly dominate the other carbon number homol- agues (cf. Fig. 8), although their side-chain alkylation (i.e. whether 23,24-dimethyl or 24ethyl) was not determined. Though present in all the samples, Cmethylsteranes were most abundant in the M-l and M-3 oil shales (Fig. 3b).

Diaslerenes and Cmethyldiasterenes. The aliphatic hydrocarbons of all samples contain t&-C, series of 20s and 20R diasterenes as significant constituents, but none contained sterenes (assessed by m/z 215). The diasterene distributions for two of the oil shales and the lignite, as seen in m/z 257 chromatograms (Fig. 10) all show a similar extent of isomerisation at C-20 (i.e. c. 44% 20s; cf. Brassell er al., 1984), but the abundance of individual carbon numbers differs significantly; specifically, C, components are markedly more abundant in the vitrinite and lignite (cf. Fig. ItIc). This latter feature is well demonstrated in the carbon number distributions of SOR-diasterenes (Fig. 9b), where the dominance of the C, components in the vitrinite is especially apparent.

The distributions of Cmethyldiasterenes in all the oil shales are similar; both 20s and 20R C&Z, components are present with the C, members dominant. The relative proportions of these Cx, compounds is significantly lower, however, in the lignite and vitrinite.

Other components. In addition to the major compound classes described above, other marker compounds diagnostic of their biological source were also present in the oil shale sequence. De-A-lupane (Corbet et al., 1980) was recognized in all the samples, but was more abundant in M-4, M-5 and M-l than in the upper oil shales. Similarly, oleanenes and noroleanenes (C,), plus minor components with mass spectra consistent with those expected for C,, and C,, neohop-13(18)-enes, were observed in the lignite and vitrinite samples and, to a lesser extent, in the basal oil shale, M-l. The lignite also contained C.&J,, tetracyclic terpanes [I7(21)-secohopanes] and a number of unidentified diterpenoids and sesquiterpenoids. These compounds were not detected in the upper oil shale horizons. The oil shales did, however, all contain three components with mass spectra (molecular ion 424, base peak ion 257) consistent

with those expected for C,, triterpenoids of femene or arborene structure with extended side chains.

Abundance of biological marker compound classes In addition to the information provided by the occur-

rence of individual biological markers, the distributions of their various classes also permits assessment of differences between samples. An indication of the relative contributions of six major compound classes in each sample was calculated from characteristic mass chromatographic responses for key ions in the spectra of n-alkanes, acyclic isoprenoid hydrocarbons, botryococcanes (all m/z 85), hopanoids (m/z 191). steroids (m/z 217 and 257) and Cmethylsteroids (m/z 231 and 271). These data are depicted as normalised histograms of compounds class abundances (Fig. 1 l), but they do not represent absolute concentrations; rather, they are intended to provide a self-consistent, albeit approximate, estimate of the relative importance of the chosen compound types to each sample. They show that only the uppermost oil shale M-M contains botryococcanes. The relative compositions of the other three oil shales are broadly comparable and significantly different from both the lignite and the vitrinite samples, especially in terms of their steroid contents. The lignite itself contains markedly higher amounts of hopanoids (cf. Fig. 2c) and steroids than the oil shales, and it is the abundance of the latter compound class which distinguishes the vitrinite. Indeed, a *major difference between the oil shales and the other lithologies is the dominance of Cmethylsteroids over steroids in the former.

A further consideration that merits attention is the relative importance of the “regular” steroid vs the ‘Ye- arranged” diasteroid skeletons among both the steroid and 4-methylsteroid compositions. In the lignite and vitrinite diasterenes are more abundant than steranes. For the lower oil shales, M-l and M-2, however, these two component classes are present in roughly equal proportions, whereas steranes are clearly dominant in the upper oil shales M-3 and M-M. A similar pattern of behaviour is seen for the Cmethylsteroids. These differences may be a function of the


1 Diasterenes 1

H

(b)

M-M ail shole

m/z 257 c,, *of?

c,, 20s

(Maaming( cIL+ Time

Fig. 10. Annotated partial m/z 257 chromatograms illustrating the distributions of diasterenes among the aliphatic hydrocarbons of: (a) M-M oil shale, (b) M-3 oil shale and (c) M-4 lignite.

clay content of the samples, since such mineral species catalyse the rearrangement process (Rubinstein et al., 1975). Alternatively, variations in the input of steroids to the sediments may be responsible.

DISCUSSION

Sample intercomparison

The oil shales show characteristics which, although similar in several respects, provide a clear distinction of the samples. M-M is unique in terms of its high content of colonial algal bodies, and it is the only sample that contains botryococcanes and the &, branched alkanes. The higher (C,?) acyclic isoprenoids, such as lycopane, are present only in M-3 and M-M. The composition of other biological mark-

ers in M-3, M-2 and M-l are generally similar, with significant amounts of Cmethylsteroids greatly dominating their steroids. M-l, however, contains components thought to derive from higher plants, such as oleanenes, which are not present in M-3 or M-2 but which do occur in the lignite. Visually, M-3 is the richest in hydrocarbon wisps and M-2 contains plen- tiful spores and algal colonies, whereas M-l has none of these characteristic properties which may, in part, reflect its markedly lower organic carbon contents (Table 1). The lignite and vitrinite are completely different, as expected, being dominated by phytoclasts, with minor amounts of resin, They differ from the oil shales in terms of their steroid vs Cmethylsteroid abundances, and from each other on their hopanoid/steroid ratios.

938 SIMON C. BRASELL et al.

(Moomlng

Fig. 1 I. Relative concentrations of biological marker compound classes in the six Maoming samples calculated from the following mass chromatographic responses for significant resolvable compounds normalised to a total of 100%: n-alkanes, sum of m/z 85 responses for C,z-C,,; acyclic isoprenoid alkanes, sum of m/z 85 responses for C,,-C,, and C,II-C20 components; botryococcanes, sum of m/z 85 responses for C,, and C,, compounds; hopanoids, total summed intensity of m/r 191 over the triterpenoid region (hopanes and hopenes); steroids sum of m/z 217 responses for C,,-Gs 5a(H)- and S/?(H)-steranes and m/z 257 responses for C,,-C, 20s and 20R diasterenes; Cmethyl- steroids, sum of m/z 231 responses for C,, and C, 4a and 4/l-methyL5a(H)-steranes and of m/z 271 responses for C,,

and C, 20s and 20R Cmethyldiasterenes.

Sources of organic matter

A major objective of this study is to deduce the sources of the organic matter in the Maoming oil shale sequence from their biological marker compositions. A second intention is to compare the information derived from the investigation of the visual kerogen with that provided by the biological markers. Such a combination of evidence should enable an accurate history of the depositional environment of the Maoming oil shale, as recorded in its organic matter, to be drawn.

Visual description. The lignite and vitrinite, as expected, contain dominantly higher plant phytoclasts, with resins. The oil shales, by contrast, contain only corroded and degraded higher plant components and spores. The dominant source of their organic matter appears to be algal, notably from dinoflagellates and colonial green algae. The information provided by the visual evidence is tangible, but imperfect. For example, such petrographic exami- nations provide little evidence for bacterial contributions to the organic matter.

Biological markers. The interpretation of the sources of sedimentary organic matter using molecular markers relies on the specificity of the biological origins of particular compounds. The dominant biological markers among the aliphatic hydrocarbons of the lignite, namely hopanoids, are not derived from higher plants, but rather originate from bacteria (Ourisson et al., 1979). Such an abundance of hopanoids in the lignite is consistent with evidence from contemporary peat deposits (Quirk et al., 1984), where hopanoids are similarly dominant. The promi- nence of pristane in the lignite and vitrinite is also recognized as a common feature of such materials (e.g. Allan et af., 1975). Terrigenous components in the lignite are represented in the dominance of higher n-alkane homologues (Eglinton and Hamilton, 1967; Simoneit, 1978) and the presence of higher plant triterpenoids (e.g. oleanene; Hoffmann et al., 1984) and their degraded derivatives (e.g. de-A-lupane; Corbet et al., 1980), diterpenoids (Simoneit, 1977) and sesquiterpenoids (e.g. Grantham and Douglas, 1980). In addition, the dominance of Cr9 components in the diasterenes is consistent with terrigenous origins for these components (e.g. Huang and Meinschein, 1979; Mackenzie et al., 1982). A similar feature is seen in the vitrinite where the C,, steroids of higher plant origin are even more pronounced.

The oil shales show various characteristics that differentiate them from the lignite and vitrinite. How- ever, the recognition of a number of terrigenous marker compounds, such as oleanene and de-A- lupane in significant amounts suggests that M-l may reflect, in part, a transitional phase between the lignite and oil shale beds, or, alternatively, some redeposition of lignite in the lower horizons of the oil shale. Other features of the oil shales are their n-alkanes of higher plant origin, and, most notably, a predominance of 4-methylsteroids from dinoflagellates. Such biological origins for 4-methylsteroids have become evident from studies of Recent marine and lacustrine sediments receiving major contributions of 4-methylsterols and 4-methylsterones from dinoflagellate algae (Boon et al., 1979; de Leeuw et al., 1983; Robinson et al., 1984) coupled with an improved understanding of the diagenetic pathways that transform such biolipids into Qmethylsteranes (Wolff et al., 1986a and b). The comparative abundance of the 4-methylsteroid components in the various samples may therefore be taken as a measure of dinoflrcgellate contributions to the sedimentary organic matter. The high concentrations of dmethyl- steroids in these oil shales are similar to those seen in several other sedimentary sequences, including the Messel shale (Kimble et al., 1974; Rubinstein and Albrecht, 1975), other Chinese oil shales (e.g. Wolff et al., 1986a and b) and Rhaetian black shales from S. W. Britain (MacQuaker et al., 1986). The dominance of CrO homologues of Cmethylsteroids in these oil shales is typical of sediments containing high amounts of such compounds and presumably reflects


the original predominance of C90 among their precur- sor 4-methylsterols (e.g. Boon ef al., 1979; de Leeuw et al., 1983; Robinson et al., 1984). The carbon number distributions of both steranes and diasterenes for the oil shales fall in the central field of the steroidal triangular compositional diagram (Fig. 9) which is non-diagnostic of either algal and/or higher plant sources. The hopanoids, as in the lignite, may be presumed to be derived from bacteria. Pristane, phytane and the lower acyclic isoprenoid aikanes probably denote contributions from either phytol (e.g. Didyk et al., 1978) or tocopherols (Goossens ef al., 1984) of photosynthetic plants. Phytane, alternatively, may originate from archaebacteria such as methanogens (e.g. Risatti et al., l984), together with lycopane (Brassell et al., 1981) and perhaps the other higher acyclic isoprenoids recognized in M-3 and M-M (i.e. peak a in Figs 3a and 3b).

The uppermost oil shale M-M also shows evidence for botryococcoid green algal contributions from the presence of C,, and C,r botryococcane homologues. Botryococcane and similar uncharacterized homologues have been previously recognized in various Sumatran crude oils (Moldowan and Seifert, 1980; Seifert and Moldowan, 1981) and in Australian shales and crude oils (R. Summons, personal commu- nication; McKirdy et al., 1986). Interestingly, C& botryococcane homologues with the same pattern of ion doublets as those seen in this oil shale were previously observed in a Sumatran crude oil (Seifert and Moldowan, 1981) Botryococcanes of any carbon number are held to be characteristic of depositional environments of brackish to freshwater facies, a suggestion consistent with the apparent depositional environment of the Maoming oil shale. The two Cr, unusually branched alkanes may be presumed, like their C& and Cl5 alkane and alkene analogues, to originate from green algae (Rowland et al., 1985). In summary, there are many individual compounds and compound classes in the various samples that are diagnostic of contributions from specific types of organisms to the Maoming sedimentary sequence, including markers for algae, terrigenous higher plants and bacterial inputs (Table 2).

Table 2. Diagnostic biological markers identified in the Maoming oil shale sequence and their inferred original biological sources

Biological markers Inferred biological source

n-Alkanes (C,,, C,,) Higher plants Botryococcanes’ Botryococcus green algae C,, Branched alkanes Green algae? Hopanoids Bacteria Lycopaneb Methanogenic bacteria? C, Diasterenes’ Higher plants 4-Methylsteroids Dinoflagellates De-A-triterpenoids Higher plants Diterpenoidsd Higher plants Sesquiterpenoidsd Higher plants Oleanene& Higher plants

OOnly recognised in the M-M oil shale. ‘Only recognised in the M-3 and M-M oil shales. ‘Only prominent in the lignite (M-4) and vitrinite (M-5) samples. dRecognised in the lignite (M-4) and vitrinite (M-S) samples, and in

lesser amounts in the M-l oil shale.

Comparison of petrographic and biological marker characteristics. Both the petrographic and biological marker data provide evidence for contributions of organic matter from higher plants and from dinoflagellate and botryococcoid algae. For these three biological sources there is general agreement between the two data sets in terms of the sample containing the greatest contribution from each type of organism; namely, higher plant, dinoflagellate and botryococcoid components are dominant in samples M-5, M-3 and M-M, respectively. There are several

differences, however, between the information on the sources of organic matter from visual description of the samples and that from the biological markers. First, the former provides little direct assessment of the bacterial contributions to the different strata. Second, visual evidence for dinoflagellates stems from the presence of wispy-like organic matter rather than discrete cysts. However, there is no correspondence between the sedimentary occurrence of dinoflagellate cysts and 4-methylsteroids, simply because they are not produced in unison. Indeed, cyst production in dinoflagellates seems to follow no regular pattern in terms of growth stages or environmental stress (Brasier, 1980). Also, 4-methylsteroids may originate from symbiotic dinoflagellates which do not encyst. In addition, some species of cultured dinoflagellate do not synthesize Cmethylsterols (Robinson, 1984). The recognition of algal colonies in the samples is independent of the occurrence of botryococcane homologues among the biological markers, an obser- vation consistent with the known variance in the occurrence of botryococcenes in different isolated strains of Bofryococcus (C. Largeau, personal com- munication). Finally, the general differences between the lignite and vitrinite and the oil shales may regulate to the abundance of amorphous materials in the samples, which, inherently, provide no precise evidence of their biological origins.

Maoming oil shale: depositional environment

Elucidation of the sources of the organic matter in the sedimentary sequence provides the suggestion of a scenario for the depositional environment of the Eocene Maoming oil shale. The basal lignite is likely to have been formed initially as a peat in shallow water. The vitrinite lens in the overlying claystone may be derived from higher plant material carried into the sediment as the waters deepened. Subsequent blooms of dinoflagellates occurred within the lake, sometimes accompanied by botryococcoid algae. The lake may ultimately have shallowed, culminating in major blooms of botryococcoid algae which finally dominate both petrographic and biological marker profiles. In several respects the occurrence and distributions of many of the biological markers, but notably the 4-methylsteroids, in the Maoming Oil shales resemble those of the Messel oil shale (Kimble et al., 1974).

940 SIMON C. BRAS~ELL et al.

Maturity of organic matter

The compositions of biological markers among the aliphatic hydrocarbons of the samples demonstrate the immaturity of the sedimentary succession and, as expected, the uniformity of its diagenetic alteration. Among the components that attest to this immaturity of the sequence are 17B(H),2l/I(H)-hopanes, A’7(21)-hopenq Al3(‘*) -neohopenes (e.g. Brassell et al.,

1980; cf. Fig. 7), diasterenes (e.g. Brassell et al., 1984; cf. Fig. lo), 4-methyldiasterenes and 4/Y-methylsteranes (Wolff et al., in press, 1986; cf. Fig. 8). In addition, the compounds typical of mature suites of biological markers, such as 20s and 14j(H),17/?(H)-steranes (e.g. Mackenzie et al., 1980, 1982) are absent in all the samples. The extent of diasterene isomerisation (c. 44% 20s) is also consistent with the maturity range typical for immature marine black shales (cf. Brassell et al., 1984).

Overall, the distributions of biological marker compounds is in general agreement with the vitrinite reflectance measurements (Table 1); namely, both are significantly lower than the oil generation threshold.

SUMMARY

Both the visual kerogen descriptions and the biological markers show differences between these immature sediment samples that can be interpreted as changes in the inputs of organic matter to the sediments being deposited within the environment. Such variability in the characteristics of outwardly similar oil shales illustrates the importance of sample selec- tion within a potential oil source bed. For example, the occurrence of the botryococcanes in only the uppermost oil shale strata might be missed in less detailed sampling, perhaps leading to erroneous correlations. The value of complementary visual and molecular descriptions of the samples is well demonstrated by the independent information that they provide. The picture that emerges from the data suggests that after an initial phase of peat deposition the Maoming lake deepened and experienced blooms of dinoflagellate algae and, ultimately, of botryococcoid algae.

In conclusion it is appropriate to add a cautionary note to the use of biological markers as sole criteria in any assessment of the sources of sedimentary organic matter. As illustrated in this paper the presence of a specific marker compound, such as botryococcane, or a family of compounds, such as 4-methylsteroids, may provide convincing evidence for contributions from their source organisms, namely botryococcoid and dinoflagellate algae, respectively, to the depositional environment. How- ever, the absence of a characteristic biological marker, or visual evidence such as dinoflagellate cysts, does not necessarily prove the absence of contributions from its source organism to the sediment, nor its absence from the depositional environment.

Acknowledgements-We are most grateful 10 Dr J. M. Jones (Newcastle University) for his visual examination of these Maoming samples and to Mrs Xu for practical assistance and to Ming Yusuan for the XRD analysis of M-2 clays. We thank the Royal Society and the Chinese Academy of Sciences for funding the collaborative research programme between the Organic Geochemistry Unit and the Institute of Geochemistry, Guiyang. We gratefully acknowledge the NERC for C-GC-MS facilitates (GR3/2951& GR3/3758) and colleagues in Bristol and Guiyang for their helpful comments, advice and technical assistance, especially Mrs A. P. Gowar and MS L. Dyas. We thank Dr C. Largeau (E.N.S.C.) for preprints and helpful discussions concerning ihe hydrocarbbn -constituents of Botryococcus, and Dr R. Summons (Bass Becking Geobiological Laboratory) and Dr D. McKiriy (Australian Mineral fievelopment Labora- tories) for information on botryococcanes in Australian sediments and crude oils.

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Biological marker compounds as indicators of the depositions! history of the Maoming oil shale

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